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Tropical Storm Isaac has now officially become Hurricane Isaac,
the National Hurricane Center announced this afternoon (Aug. 28).

This strengthening had been expected for more than 24 hours,
however — so why did it take so long?

Tropical Storm Isaac was still intensifying early yesterday
(Aug. 27) when it encountered a system of dry air, said Tim
Schott, a hurricane specialist with the National Oceanographic
and Atmospheric Administration. The dry air was sucked into the
cyclone and prevented the eye wall (the border surrounding the
cyclone's center, or eye) from forming, a key step in a storm's
intensification, he told OurAmazingPlanet.

It's also a
large storm, with tropical storm force winds extending for
nearly 200 miles (320 kilometers). Larger storms sometimes do not
intensify as quickly as smaller, more compact cyclones, Schott
said.

But whether or not it passed the hurricane threshold doesn't
really matter, he said — either way it would be a very large
system that will have wide-ranging effects. "Everybody likes to
put everything into neat little boxes, but it doesn't matter that
it's intensified from 70 to 75 miles per hour. The difference is
really not very consequential."

The cyclone finally achieved hurricane strength this afternoon
and currently has winds of 75 mph (120 kph), just over the
hurricane threshold, according to the National Hurricane Center.

It could intensify slightly in the next 12 to 18 hours, but not
by much, according to the NHC. Isaac has limited time and room
over ocean waters before it makes landfall, after which it will
begin to weaken.

The dry air has weakened the storm's pattern of convection, the
rising air that forms the thunderstorms that make up a tropical
cyclone. The dry air is still within the system, and may prevent
it from strengthening as much as it otherwise would have.

The storm is currently moving to the northwest at 10 mph (16 kph)
and the center of the storm could make landfall as early as this
evening. It's expected to produce a storm surge between 6 to 12
feet (1.8 to 3.7 meters) in Mississippi and southeastern
Louisiana.

As of the latest update from the U.S. National Hurricane Center,
Isaac's center was located about 165 miles (260 km) southeast of
New Orleans.

A hurricane warning is currently in effect from east of Morgan
City, La., to the Mississippi-Alabama border, an area that
includes New Orleans. The storm is bearing down on New Orleans
seven years to the day after Hurricane Katrina devastated the
city.

Hurricane watches and tropical storm warnings and watches have
been issued for other areas of the Gulf Coast. The NHC has warned
that the impacts from Isaac's
storm surge are likely to be significant and that the storm
could bring large amounts of rain, as well as the possibility of
tornadoes in some places.